1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a steel cord of a specified structure for use as a reinforcing material of rubber products such as radial vehicle tires and conveyor belts.
2. Description of the Related Art
Steel cords are commonly used as a reinforcing material in rubber products such as radial vehicle tires and conveyor belts.
These steel cords are generally made by twisting together a plurality of steel wires, but if they are not well integrated with the rubber matrix they cannot function fully as a reinforcing material. Also, when the rubber matrix does not penetrate well into the inside of the steel cord, water enters spaces inside the steel cord not filled by the rubber matrix, and as a result corrosion spreads and the life of the rubber composite itself also falls.
To solve this kind of problem, in Japanese Patent Publication No. H.5-186978 there is disclosed a steel cord of a 1+6, 1+7 or 1+8 structure which comprises six to eight sheath steel filaments (hereinafter referred to as sheath filaments) around a single core steel filament (hereinafter referred to as a core filament) preformed into a two-dimensional wavy form and of which the sectional shape perpendicular to the length direction of the cord is elliptical.
FIG. 8-A and FIG. 8-B show this steel cord; in these figures, co is a core filament preformed with a two-dimensional wavy form and ou is a sheath filament. In a steel cord of this structure, as a result of the wavy form of the core filament co, gaps are formed between the filaments and rubber can penetrate easily into the cord. And, because the sectional shape of the cord is flat and is uniform in the cord length direction, it is also easy to work during calender processing and there is the merit that the gauge thickness of rubber into which it is embedded can be made thin.
However, in this related art, there is no disclosure of a method for manufacturing a steel cord having these characteristics. A general method for manufacturing a steel cord of this kind of I+n structure uses a tubular twisting machine. That is, a zigzag-shaped preform is formed in a core filament, sheath filaments each having a predetermined amount of preform are wound around the core filament with a tubular twisting machine (at this time, the filaments are not twisted), and after that the sectional shape of the steel cord is formed into an elliptical shape by means of a flattening device such as correcting rollers. However, with methods that use a tubular twisting machine there has been the major problem that productivity is poor and the product cost consequently is high.
As a way of overcoming this it is convenient to manufacture a cord with a double-twist (bunching) type twisting machine. With methods that use a bunching type twisting machine, since two twists can be made in a single rotation, the manufacturing efficiency is high and the product cost can be reduced.
However, in a bunching type twisting machine, in the twisting process, all the filaments are twisted together at once in a bundled state. Because of this, the sectional shape of the steel cord collapses and the flat faces (the major axis sides) fail to align in a fixed direction and the gaps between the sheath filaments become non-uniform, and as a result stability of shape, which is an important characteristic of a steel cord, is lost and also the rubber penetration becomes unstable. Consequently, it has been difficult for the steel cord to be brought into use industrially.